1. Field of the Invention
The present invention generally relates to the field of optoelectronic image sensing, and more specifically to a photodetector or other photoresponsive device including a graded composition photoresponsive layer and recessed contacts to trap minority carriers at the surface of the photoresponsive layer and increase the excess carrier lifetime and thereby the photoresponsivity of the device.
2. Description of the Related Art
Mercury-cadmium-telluride (HgCdTe) photoconductors are widely used in infrared (IR) imaging systems, especially at wavelengths between 8 and 12 micrometers. HgCdTe is a ternary solid solution having the composition Hg.sub.1-x Cd.sub.x Te, where x is the mole fraction of cadmium telluride (CdTe). Photodetectors fabricated from this material generally include an n-type photoresponsive layer of HgCdTe formed on a substrate of CdTe or mercury-zinc-telluride (HgZnTe), with laterally spaced electrical contacts attached to the surface of the photoresponsive layer.
The excess minority carrier lifetime is an important characteristic of these devices. Increasing the minority carrier lifetime reduces the carrier recombination rate, thereby increasing the responsivity of the device. U.S. Pat. No. 4,914,495, entitled "PHOTODETECTOR WITH P LAYER COVERED BY N LAYER", issued Apr. 3, 1990 to P. Norton et al discloses a Heterojunction Interface Trap (HIT) photoconductive device fabricated from HgCdTe by liquid-phase epitaxy (LPE). The HIT device includes an n-type IR photoresponsive layer formed on an electrically floating p-type layer located between the n-type layer and a CdTe substrate. The n-type and p-type layers form a heterojunction therebetween.
Minority carrier holes which are photogenerated in the n-type layer diffuse into the p-type layer, and are trapped therein by the potential barrier created by the heterojunction. Minority carrier hole recombination is greatly reduced by the trapping effect, thereby substantially increasing the responsivity of the HIT device compared to a conventional photodetector.
However, a certain amount of recombination does occur in the HIT device at the negative contact, thus limiting the responsivity enhancement of the structure.